1. Field of the Invention
The present invention relates to electric-power controllers generally and more specifically to a fluorescent-lighting-system voltage controller in which auto-transformer taps are switched in closed-transition fashion while the auto-transformer neutral is opened.
2. Description of the Prior Art
When operated at full voltage, many fluorescent (and other gaseous-discharge-type) lighting systems generate lighting levels in excess of those which may be required at a particular time and/or for a particular task. For example, reduced levels may be suitable for janitorial and/or security purposes. Also, reduced levels may be appropriate when suitable levels of natural lighting are available. Further, since the lighting levels (and efficiencies) of fluorescent lamps decrease as the lamps age, many systems may be operated at reduced levels when relatively new lamps are employed therein.
During lamp starting, it is important that fluorescent-lighting systems be operated at full voltage. Thereafter, the voltage may be reduced to reduce lighting levels and power consumption. in general, the power-consumption reduction is in direct proportion to the voltage reduction; and, the reduction in lighting levels is in a linear proportion to the voltage reduction.
A fluorescent-lighting-system-starting and one, or more, reduced voltages are developed by a prior-art-type voltage controller which includes a motor-driven variable transformer. Unfortunately, motor-driven variable transformers suitable for use in the above-mentioned controller are relatively expensive.
Another prior-art-type voltage controller for use with a fluorescent-lighting system is disclosed in the U.S. Pat. No. 4,219,759 which issued to R. Hirschfeld. The controller includes three auto-transformers each connected between a neutral line and a respective one of three hot lines (Y-connection), the lines for connection to a three-phase four-line (AC-voltage-supplying) power system. Each of the auto-transformers has a tap at which the auto-transformer develops a fluorescent-lighting-system-starting voltage and another tap at which the auto-transformer develops a reduced voltage. Also included in the controller are a pair of three-pole magnetic contactors (relays), the contactors for selectively coupling the auto-transformer developed voltages to three additional hot lines, which, with the above-mentioned neutral line, are for connection to the fluorescent-lighting system (as a load). More specifically, each of the three lighting-system hot lines is selectively coupled by a respective set of normally-open contacts of one of the contactors to a respective one of the starting-voltage taps. Additionally, each of the three lighting-system hot lines is selectively coupled by a respective set of normally-open contacts of the other contactor to a respective one of the reduced-voltage taps.
A voltage controller for single-phase systems is disclosed in the U.S. Pat. No. 4,189,664 which also issued to R. Hirschfeld.
Controllers of the type disclosed in the above-mentioned patents are disadvantageous in that they require critical contactor timing. During any period in which the contacts of both contactors are simultaneously closed (make-before-break) a fault condition occurs (as the auto-transformers are shorted). Alternatively, during any period in which the contacts of neither contactor are closed (break-before-make), the power to the fluorescent-lighting system is interrupted (power outage). Such an interruption may cause certain lamps, particularly mercury-vapor-type lamps, to extinguish.
A controller which avoids the above-mentioned power-interruption problem includes three switching reactors in addition to three auto-transformers (or a transformer with three auto-transformer windings) and two contactors, the switching reactors being interposed between the contactors. More particularly, each of the switching reactors has an end which is selectively coupled by a respective set of contacts of one of the contactors to a respective auto-transformer-starting-voltage tap; another end which is selectively coupled by a respective set of contacts of the other contactor to a respective auto-transformer-reduced-voltage tap; and a center tap which is connected to a respective one of three lighting-system hot lines. To avoid power interruptions, the contactor timing is overlapped (make-before-break) to provide what is referred to as "closed-transition switching." During switching, the switching reactors are operative to limit the auto-transformer currents. Unfortunately, the limited currents developed during switching are high (fault level), inductive currents, under which the contactors must open.
Some electric motors are started under reduced voltage. A circuit which provides such a reduced voltage for motor starting and full voltage thereafter is commonly referred to as the Korn-Dorfer method. For three-phase motors, such a circuit includes a transformer having three auto-transformer windings (or three auto-transformers each having one winding). The hot end of each winding is connected to a respective one of three hot lines for connection, with a neutral line, to a three-phase four-line power system. The neutral end of each winding is selectively coupled by a respective set of (neutral) contacts to the neutral line. A reduced-voltage tap on each of the windings is selectively coupled by a respective set of (reduced-voltage) contacts to a respective one of three motor hot lines each of which is also selectively coupled by another respective set of (full-voltage) contacts to a respective one of the three hot power lines. During starting, the neutral contacts and the reduced-voltage contacts are closed; and, the full-voltage contacts are open. After starting, the neutral contacts are opened. Thereafter, the full voltage contacts are closed. Finally, the reduced-voltage contacts are opened.